In orthopedic, oral and craniofacial surgery, there is often a need for a graft material to repair bone defects originating from disease, surgery, or traumatic injury. The primary bone graft material used consists of human bone tissue collected either from the patient or from a donor. Autogenous bone from the patient has been demonstrated to induce bone growth and defect repair, but its collection generally requires a second surgical site, increasing the risk of complications and recovery time. Allogenic bone from donors or cadavers also is used, but these tissues may result in immunologic reactions and rejection and harbor the potential for disease transmission. Beyond the medical problems associated with these tissue grafts, another problem is tissue graft form. When supplied as a piece of solid bone, these grafts can act as structural grafts to support physiologic loads. However, these grafts are difficult to shape and anchor in place without the use of surgical pins or screws. To overcome this problem, it is known to prepare tissue grafts in a paste or putty form to fill non-uniform defects, but these grafts remain in a soft pliable form and are not load-bearing without the use of other structural surgical hardware such as pins, screws, and bone plates.
Another common problem with tissue grafts is their unpredictable absorption properties. Some tissue grafts may last several years in a site, while otherwise similar tissue grafts may be absorbed within several months. In certain surgical procedures, this absorption rate may result in compromised healing if the graft is absorbed too rapidly. In such cases, very slow absorption and strong mechanical strength are required to maintain long-term mechanical support. An example of such a procedure would be in vertebroplasty surgery, in which a needle is inserted into the center of a collapsed vertebra and a bone replacement material is injected. When the material has filled the bone cavity, it is cured and set, thereby stabilizing the fractured vertebra. As bone growth in such sites is very limited, long term support from the injected material is critical.
Synthetic graft materials, those not involving human and/or animal tissues, also are used for bone defect filling, but primarily as bulk inactive fillers. The main problem with most synthetic materials is that they act solely as passive scaffolds for bone repair and do not stimulate bone formation similar to autogenous bone. Another problem with these materials is that they primarily are in a particulate or paste form, hence they suffer from the same problem as the putty/paste forms of tissue grafts in that they are not load-bearing and require use of other structural hardware. Another drawback associated with many synthetic materials is that they have absorption rates incompatible with bone healing. Some materials such as metals and hydroxyapatite ceramic/cements are permanent replacements and impair healing of the graft site. Other materials such as calcium sulfate ceramics are absorbed by the body at a rate faster than bone formation can occur, leaving a weakened graft site.
One synthetic material has been demonstrated to directly stimulate the cells necessary for bone formation. This material, bioactive glass, is generally composed of the elements silicon, calcium, phosphorus, sodium, and oxygen, although other elements such as boron, potassium, magnesium and fluorine for example, may be added to modify various characteristics, as disclosed in U.S. Pat. Nos. 4,103,002, 4,775,646 and 4,851,046, the disclosure of which is incorporated herein by reference. A representative bioactive glass composition may comprise for example 40 to 52 wt. % SiO2, 10 to 50 wt. % CaO, 10 to 35 wt. % Na2O, 2 to 8 wt. % P2O5, 0 to 25 wt. % CaF2, 0 to 10 wt. % B2O3, 0 to 8 wt. % K2O, and 0 to 5 wt. % MgO. As a preferred example, one specific bioactive glass composition, marketed under the brand name BIOGLASS®, has a composition of approximately 21% silicon, 18% calcium, 18% sodium, 3% phosphorus, and 40% oxygen (by weight percent). This BIOGLASS material has been used clinically for over 12 years as a particulate bone graft composition.
Bioactive glass bone grafting compositions have been defined as being osteostimulative, stimulating the function of the osteoblast cells responsible for bone formation. The osteostimulative action of the material is a function of the material composition and its dissolution and absorption characteristics. Upon implantation and contact with body fluids, the bioactive glass particles begin to react, dissolving out sodium, calcium, and phosphorus ions from the surface. The calcium and phosphorus ions redeposit back onto the surfaces of the composition particles, forming a calcium-phosphate layer similar to the hydroxylapatite mineral that makes up the natural mineral phase of bone. Osteoblasts and other proteins in the graft site readily attach to this calcium-phosphate layer, resulting in the rapid osteostimulative bone formation directly in contact with the composition. Over time, the surface dissolution and reactive processes continue until the entire composition is absorbed, leaving behind only the newly formed bone.
One of the drawbacks with bioactive glass bone graft compositions is that they only are available in particulate or non-hardening paste forms, which limits their practical application in load-bearing sites. In addition, the absorption rate of this bioactive glass is such that it may be absorbed prior to complete healing of defects requiring extended healing times, such as in the aforementioned spinal defects or in subjects with compromised and/or delayed healing rates. As with the tissue grafts, a very slow absorption rate and strong mechanical strength are required to maintain long-term mechanical integrity. For these cases, it is desired that the graft material stay active in the graft site for longer than periods than currently achieved to prolong the stimulatory action on the bone.
In these respects, the osteostimulative bone graft putty according to the present invention substantially departs from the conventional concepts and designs of the prior art, and in so doing, provides a composition developed primarily for providing a load-bearing graft material that stimulates bone formation and healing as it is absorbed from the implant site.
In view of the foregoing disadvantages inherent with the known types of graft material present in the prior art, the present invention provides a new bone graft material that can be readily applied to a graft site to support local applied loads and stimulate new bone formation and defect healing.
The general purpose of the present invention, which will be described subsequently in greater detail, is to provide a new bone graft material that has many of the advantages of the graft material mentioned heretofore and many novel features that result in a new bone graft material which is not anticipated, rendered obvious, suggested, or implied by any of the prior are bone graft materials, either alone or in any combination thereof.
In this respect, before explaining at least one embodiment of the invention in detail, it is understood that the invention is not limited in its application to the details of composition or the proportions thereof as set forth in the following description. The invention is capable of other similar embodiments. Also, it is to be understood that the phrasing and terminology employed herein are for the purpose of the description and should not be regarded as limiting.
The primary object of the present invention is to provide a bone graft material that will overcome the shortcomings of prior art compositions.
An object of the present invention is to provide a bone graft material that will set after implantation and act as a load-bearing material.
Another object is to provide a bone graft material that is not permanent and will be absorbed from the graft site.
Another object is to provide a bone graft material that will stimulate bone formation in the graft site immediately upon implantation.
Another object is to provide a bone graft material that will sustain bone stimulation at extended periods after implantation.
Other objects and advantages of the present invention will become obvious to the reader and it is intended that these objects and advantages are within the scope of the present invention. To accomplish the above and related objects, this invention is described in the detailed description presented herewith, attention being called to the fact, however, that the variations to the actual details may be made in the course of final development work.